1. Field of the Invention
This invention is in the field of burners for introducing a suspension of solid fuel particles into a combustion chamber such as a rotary kiln. It provides an arrangement of concentric conduits which create substantial turbulence in the particle suspension and combine it with a secondary air source to provide efficient combustion in the combustion chamber.
2. Description of the Prior Art
The present invention is particularly applicable to providing a burner assembly for the introduction of solid fuels into a rotary tubular kiln. In such rotary kiln burners, the air necessary for combustion is partly derived from the air stream which conveys the fuel particles into the chamber and is partly composed of hot exhaust air. This air source is called primary air. A source of secondary air is combined with the primary air, the secondary air being derived from a cooler following the rotary tubular kiln. Typically, such secondary air has a temperature of 800.degree. C. or more so that this secondary air cannot be employed for the transport of the solid fuel, particularly coal dust, for reasons of safety. For reasons of thermal economy, the proportion of primary air must be kept as small as possible.
Rotary kiln burners for coal dust are described in the periodical "Zement-Kalk-Gips" (Vol. 32 (1979) No. 8, pages 386-389) with particular reference to FIG. 4, which illustrates a burner in which a number of tubes are disposed coaxially, one inside the other, whereby a mixture consisting of primary air and coal dust leaving the burner axially or with a slight divergence is surrounded at both sides in a radial direction by a stream of clean air. The inside air stream, as seen in the radial direction, emerges from the burner with an arcuate flow pattern, and the outer air stream emerges from the burner by means of axial bodies or with a slight divergence. The shape of the flame is controlled by controlling the outer and/or the inner air stream independently of the discharge rate of the mixed stream which is largely determined by the primary air stream.
In achieving a complete and rapid combustion, however, the problem of securing an intimate admixture of the mixed stream consisting of primary air and solid fuel emerging from the burner on the one hand with the hot secondary air on the other hand arises. Fundamentally, this can be achieved by means of providing a sufficient turbulence at the emerging mixed stream. However, there are difficulties in accomplishing this condition. There are limits on the amount of increase of discharge rate of the mixture, since a flame in the rotary tubular kiln which is too long has a considerably disruptive effect on the pyrometric process, particularly on the location of the sintering zone and the calcining zone of the rotary tubular kiln. The lower limit of discharge rate is dependent on the conditions under which the powdered material is conveyed into the rotary kiln. There is an urgent need for varying the amount of fuel introduced into the rotary kiln and thus, the amount of heat, since there are considerable fluctuations with respect to calorific content and other parameters influencing the combustion process in the use of solid fuels in contrast to liquid or gaseous fuels. It is, therefore, necessary to introduce variable amounts of fuel into the rotary tubular kiln and at the same time maintain the conditions for a thorough intermixing of the mixture and secondary air to such a degree that the flame which occurs remains short, concentrated and hot.
The use of a spiraling or helical motion in the emerging coal dust-air mixture has been considered in order to be able to better mix the fuel with the secondary air in the rotary kiln. This, however, is not satisfactory because the coarser coal dust particles have too short a flight path and strike the material to be roasted or the wall of the kiln in a state of incomplete combustion.